Typical compressed gas systems for filling one or more vehicles are fast-fill, time-fill, or combination-fill. A fast-fill compressed gas system dispenses compressed gas at a rate similar to what a user would experience using a conventional gasoline or diesel system. To achieve this rate of fill, a fast-fill compressed gas system utilizes one or more storage tanks and compressors to dispense compressed gas to vehicles. A time-fill compressed gas system is designed where a longer fueling window is available, typically overnight from 4 to 12 hours. In a time-fill compressed gas system, vehicles are generally filled directly from one or more compressors, not from storage banks. A combination-fill compressed gas system combines features of both fast-fill and time-fill systems, thereby having the ability to fast-fill vehicles throughout the day and also time-fill vehicles overnight.
There are multiple types of fast-fill systems including cascade fast-fill systems and buffered fast-fill systems. Cascade fast-fill systems are commonly used in retail applications and where vehicles arrive randomly for refueling. Cascade fast-fill systems are designed to have enough storage capacity to handle peak fuel demand with one or more appropriately sized compressors to meet the fueling demand and adequately replenish the compressed gas storage over a given period. Unlike cascade fast-fill systems, buffered fast-fill systems dispense directly from the compressor into one or more vehicles and have a smaller quantity of storage. Buffered fast-fill systems are commonly utilized for captive fleets for sequential fueling of high fuel use vehicles one after another, such as taxis and transit buses. The storage is designed to allow the compressors to operate while the vehicles are being interchanged.
In the United States, the industry standard pressure rating for compressed natural gas for a user's vehicle tank is 3,600 psig at 70° F. While 3,600 psig at 70° F. is the target vehicle tank pressure, this could change significantly based on vehicle design and the region or country of operation. For example, some systems in the United States and other systems in other countries are only rated to 3,000 psig. Vehicle tanks are typically designed to be filled to 125% of their rated pressure. Thus, a 3,000 psig vehicle tank can be safely filled to approximately 3,750 psig, while a 3,600 psig vehicle tank can safely be filled to approximately 4,500 psig. Most compressed gas systems would not fill a 3,600 psig rated tank past approximately 4,100 psig, since the desired tank pressure is approximately 3,600 psig at 70° F.
A typical cascade fast-fill system has one or more compressors for compressing gas and three storage banks: a low storage bank, a middle storage bank, and a high storage bank. Outside the United States, a control system typically directs the flow of compressed gas from the low, middle, and high storage banks to one or more compressed gas dispensers, while in the United States, one or more dispensers typically direct the flow of compressed gas from the low, middle, and high storage banks to one or more vehicles located at the one or more compressed gas dispensers. A control system, commonly referred to as a “priority panel,” typically operates so that the low storage bank conveys a storage fill portion of compressed gas through the control system to a vehicle tank. As used herein, the terminology “storage fill portion of compressed gas” is intended to describe that portion of the compressed gas from the one or more compressors that is either stored in one or more of the storage banks or that portion of the compressed gas stored in one or more storage banks that is dispensed through the control system to one or more vehicles. The control system or priority panel or dispenser operates so that the low storage bank terminates fill upon a predetermined characteristic, such as a minimally accepted flow rate or a substantially equalized pressure between the low storage bank and the vehicle tank. Minimally accepted flow rate could be utilized since as the storage bank pressure and the vehicle tank pressure equalize, the pressure differential between the storage bank pressure and the vehicle tank decreases, causing the flow rate to decrease. For example, the low storage bank may terminate fill at approximately 2,000 psig. However, this 2,000 psig value can significantly vary, as this pressure is highly dependent upon a number of variables including the tank pressure at fill initialization, the pressure of the gas supply, and environmental factors.
After the control system or priority panel or dispenser terminates the fill from the low storage bank, the control system or dispenser then operates to convey a storage fill portion of compressed gas from the middle storage bank, through the control system, and to the vehicle. The control system or the dispenser allows gas to flow from the middle storage bank to terminate fill upon a predetermined characteristic, such as minimally accepted flow rate or substantially equalized pressure between the middle storage bank and the vehicle tank. In this embodiment, the middle storage bank may terminate fill at approximately 3,000 psig. However, like the 2,000 psig termination of the low storage bank, this value can significantly vary, as it is dependent upon a number of variables.
After the control system or priority panel or dispenser terminates the fill from the middle storage bank, the control system or dispenser then operates to convey a storage fill portion of compressed gas from the high storage bank, through the control system, and to the vehicle. The control system or dispenser then operates the high storage bank to terminate fill upon a predetermined characteristic, such as a minimally accepted flow rate or a substantially equalized pressure between the high storage bank and the vehicle tank. In this embodiment, the high storage bank may terminate fill at approximately 3,400 psig, for example. Like the 2,000 psig termination of the low storage bank and the 3,000 psig termination of the middle storage bank, this value can significantly vary, as it is dependent upon a number of variables. Since a full tank pressure is approximately 3,600 psig at 70° F., the vehicle tank may then be directly filled. Direct fill means that a direct fill portion of the compressed gas is conveyed from at least one compressor through the control system or priority panel and directly to the vehicle being filled. The terminology “direct fill portion of compressed gas,” as used herein, is intended to describe that portion of the compressed gas from the one or more compressors that is not stored in any storage bank arrangement, but rather, is dispensed directly through the control system to one or more vehicles.
A buffered fast-fill system utilizes a single storage bank. The storage bank could be a single storage tank or the storage bank could have multiple storage tanks. The control system conveys a storage fill portion of compressed gas from the storage bank to the vehicle through the control system. The control system operates the storage bank to terminate fill upon a predetermined characteristic. Since a full tank fill is approximately 3,600 psig at 70° F., the vehicle tank could then utilize direct fill.
Currently, in either a cascade fast-fill system or a buffered fast-fill system, a problem arises when multiple vehicles having differing tank pressures seek compressed gas from the same storage bank, or when multiple vehicles having differing tank pressures seek to be directly filled. For example, when two vehicles having differing tank pressures seek compressed gas from the same storage bank, a vehicle having a lower tank pressure receives a greater percentage, if not 100 percent, of the compressed gas than the vehicle having a higher tank pressure. This is because, without external influence, compressed gas flows from a region of higher pressure to a region of lower pressure. As a result, when compressed gas flows from the same storage bank to the vehicle having a lower tank pressure, the flow of compressed gas to the vehicle having a higher tank pressure can greatly decrease or even stop. Logically, the user of the vehicle having a higher tank pressure may become frustrated that, for at least a period of time, little or no compressed gas is being dispensed to the user's vehicle. This period of time can last from a few seconds to a few minutes.
Additional problems may result where little or no compressed gas is dispensed to a vehicle having a higher tank pressure. The user of a vehicle having a higher tank pressure may prematurely terminate the fill by manually turning off or disconnecting from the dispenser, or the dispenser may automatically terminate the fill where little or no compressed gas is conveyed to a vehicle having a higher tank pressure. Further, many compressed gas dispensers obtain data during the fill to create target fill pressures based on ambient temperature and the amount of gasoline gallon equivalents dispensed. By terminating the fill early, the dispenser loses these set points.
Even further, when a user attempts to top off the prior incomplete fill, the user is not usually able to obtain a complete fill. As an example, on a warm day, the dispenser dispenses compressed gas to the vehicle until the tank reaches approximately 4,000 psig. Over time, as the compressed gas cools, the pressure of the compressed gas decreases to the target pressure of 3,600 psig at 70° F. However, if the fill is terminated prematurely either by the user or by the dispenser, a complete fill is difficult for a period of time thereafter, since the dispenser assumes the compressed gas within the vehicle tank is at ambient temperature.
Therefore, there is a need for a compressed gas delivery system that eliminates or greatly decreases the period of time in which little or no compressed gas is dispensed to a vehicle having a higher tank pressure to provide a continuous fueling experience.